Stereo radio receiver

ABSTRACT

A stereo radio receiver has a channel separation stage for setting an optimal channel separation of the left audio channel from the right audio channel, even under sufficient receiving field strength following a time of multipath reception for the duration of the operating time of a timing element, to a reduced (i.e., lesser degree of) channel separation.

BACKGROUND INFORMATION

Modern-day stereo transmitters provide, on the transmitter side, for asignal format which emphasizes the base width of the stereo signal andthe channel separation in order to enhance the audibility of thethree-dimensional sound of a stereo transmission.

This benefit that is associated with the reception of present-day stereotransmitters is lost when the transmitters reach a reception locationwith only little field strength. This is due to the fact that, in orderto have an interference-free channel separation, a minimumfield-strength value of the stereo transmitter is required at thereceiving location. This is because expensive stereo radio receivers, inparticular, car radios, have come equipped for quite some time alreadywith a phased transition from stereo to mono reproduction, which isdependent on the receiving field strength. Therefore, in car radios ofthis kind, the channel separation is automatically varied in response toa drop in the receiving field strength during travel.

Often, however, besides the fluctuation in the receiving field strength,in order to receive the tuned transmitter, reception areas, which arewell serviced by the tuned transmitter, a multi-path reception should beused. An inconvenience that becomes noticeable under such receptionconditions is that, even when using the stereo radio receivers describedabove, with their phased mono-stereo transition tuned to optimal channelseparation, one experiences a sudden disruption in the satisfactorystereo reproduction, often only on a short drive into a multi-pathreception area. For that reason, for the multi-path reception of thetuned transmitter, stereo radio receivers come equipped with a detectorwhose output signal controls a changeover switch for switching thereceiver to monophonic reproduction.

On travel distances where one encounters brief multipath reception, forexample on streets with multi-story buildings or in mountainous regions,the switch is often made between single-path and multi-path receptionwithin one to two seconds, and thus, in a corresponding frequency,between stereo and monophonic reproduction of a stereo transmission. Forsome time now, drivers have perceived this effect to be so disturbingthat, in mountainous regions, they turn off the stereo decoder, whenreceiving present-day stereo transmitters, and do entirely without thestereo reproduction.

U.S. Pat. No. 5,432,854 describes a conventional FM stereo receiver,which, as a function of reception noises isolated by a high-pass filterfrom the stereo multiplex signal, including among these multi-pathreception as well, generates a control signal for reducing the stereochannel separation of an audio signal to be reproduced, in such a waythat in response to a reception noise, it reduces the stereo channelseparation and, after expiration of a predefined time period, after thereception noise has decayed, raises it again to its original value.

An arrangement is described in European Patent Application No. 0 030 874for reducing the influences of reception noises on a reproduced audiosignal, in the case of which, from reception noises, a control signal isderived in an analogous manner, with which the band width of thereproduced audio signal is influenced in response to the occurrence of anoise interference.

SUMMARY OF THE INVENTION

These types of disturbances are able to be substantially reduced byusing a stereo radio receiver according to the present invention, itbeing possible, outside of the duration of the actual disturbances, tohave an at least partially stereophonic reproduction, even if it is withreduced stereo channel separation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows stages of a conventional stereo radio receiver,supplemented by a timing element for controlling a channel-separationstage in a stereo decoder.

FIG. 2 shows a timing element in conjunction with a phase shifter for achannel separation in a MPX signal path of the stereo decoder.

FIG. 3 shows a representation of the charging profile of a capacitor inthe timing element.

FIG. 4 shows the timing element in conjunction with a circuit for aphased mono-stereo switchover.

DETAILED DESCRIPTION OF THE INVENTION

Via an antenna 1, the stereo radio receiver shown in FIG. 1 receives afrequency spectrum, within which also lie the stereo transmitters ableto be received at the particular moment. A tuner 2 tunes the stereoradio receiver to the frequency of the desired stereo transmitter. Theoutput of tuner 2 is linked to an intermediate frequency stage 3, wherethe transmitter modulation is translated to the intermediate frequencyof the receiver, and then demodulated by the intermediate frequency. Ata first output 4 of ZF stage 3, the transmitter modulation, theso-called MPX signal, is adapted to be tapped off. A second output 5 ofZF stage 3 always supplies a signal in response to the carrier frequencyof the tuned transmitter reaching antenna 1 over a plurality of paths,thus in response to a so-called multi-path reception.

The two stereo signals contained in the MPX signal are separated in astereo decoder 6, and fed into associated audio channels 7 and 8. Eachaudio channel includes an NF amplifier and a loudspeaker for making theparticular stereo signal audible.

Stereo decoder 6 includes, among other things, a channel-separationstage (stero-separation) 9 and an automatic (controlled) mono-switchingstage 10. The automatic mono-switching stage 10 effects the monophonicreproduction of the actually available stereo signals via both audiochannels 7 and 8. It is always operatively switched, then, by a detector11 when the detector senses a control signal of sufficient magnitude atsecond output 5 of ZF stage 3, indicating, as described above, amulti-path reception of the carrier frequency of the tuned transmitter.

When working with a multi-path reception of this kind, phase differencesexist among the carrier frequencies of the stereo transmitter on thedifferent paths loading to antenna 1, the phase differences degrading,among other things, the separability of the two stereo signals. Theautomatic mono-switching stage 10 avoids disturbances of this kindproduced in the separated channels, which can degrade reproductionquality.

The stereo radio receiver described above is conventional, above all, incar radios, so that there is no need to describe the individual modulesin detail.

In addition to automatic mono-switching stage 10 at the output ofdetector 11, the stereo radio receiver shown in FIG. 1 has a timingelement 12, which is linked to a control input of channel-separatingstage 9.

As shown in FIG. 2, channel-separating stage 9 can be configured as acontrollable phase shifter that is linked to MPX output 4 of ZF stage 3.

From conventional methods, it is possible to permanently set phaseshifters or to adjust them at the time of manufacturing in the MPXsignal path. To effect an optimal channel separation, the phase shiftersare set up at the time the circuit is dimensionally designed oradjusted.

To avoid the disturbances described above, in this embodiment of thepresent invention, the phase shifter is designed to be controllable by atiming element, and is dimensionally designed so that during theoperating time of the timing element, the optimal (i.e., optimizationof) channel separation is canceled and is retained to a lesser extent.

A phase shifter of this kind includes, for example, a series resistor R₁of 82 kOhm connected to the MPX output of the (ZF-IC) TDA 1593, acapacitor C₁ of 27 pF connected to ground, a further series resistor R₂of 100 kOhm, and a second capacitor C₂ of 330 pF connected to ground.The output of this phase shifter leads to the MPX input, and to PIN 20of stereo decoder TDA 1592.

In parallel to capacitor C₁, a third capacitor C₃ of 82 pF is connectedto series resistor R₁. The base point of capacitor C₃ is connected tothe collector of a transistor T₁, for example of a BC 848 C, whoseemitter is connected to ground.

Included in the basic circuit of transistor T₁ is a series resistor R₃of 27 kOhm, which is followed by a cross (shunt) resistor R₄ of 190kOhm, which is connected to ground. Disposed in parallel to resistor R₄is a fourth capacitor C₄ of 22 μF. The high point of capacitor C₄ isconnected via a series resistor R₅ of 1 kOhm to the cathode of adecoupling diode D₁, e.g., N4148, which is linked to the output ofdetector 11.

In response to a critical magnitude of the multi-path reception, adetector signal appears at this output, forcing transistor T₁ to openwith time constants R₅/C₄. As a result, capacitor C₃ is switched inparallel to capacitor C₁, thereby reducing the optimal (optimization of)channel separation. The specified values are used to set the possiblecrosstalk acceptable between the channels to 12 dB. In response to achange in the values the extent of which the channel separation isreduced can be varied.

At the same time, however, the automatic mono-switching stage 10 (e.g.,via PIN 16 at IC 1592) is energized, so that the reduction in thechannel separation does not become noticeable during the multi-pathreception itself. If the vehicle equipped with the stereo radioreceiver, for example, as a car radio, leaves the multi-path receptionarea of the tuned transmitter, then the detector signal is fails toappear. This deactivates automatic mono-switching stage 10. At thisinstant, however, the potential drops again across the anode of diodeD₁. However, transistor D₁ is again switched through further in responseto the charging of capacitor C₅. At this point, the capacitor is slowlydischarged across cross resistor R₃ and still keeps transistor T₁ openfor a certain time span. The length of this time span is defined by thetwo components C₄/R₄, which make up the timing element. When the timingelement's operating time elapses, transistor T₁ slowly passes over intothe blocked state. This causes the channel separation effected by thephase shifter to increase continually to the optimal value.

When, during the operating time of timing element C₄/R₄, in whichtransistor T₁ is fully open, the vehicle drives into a new multi-pathreception area of the tuned transmitter, then the detector signalappears again and recharges capacitor C₁.

FIG. 3 shows the charging profile of capacitor C₄, at whose upperlimiting value transistor T₁ is completely open, and at whose lowerlimiting value the blocking of transistor T₁ begins. As shown in FIG. 3,the charging of capacitor C₄ is initiated by the first-time occurrenceof the detector signal. At the end of the detector signal, the operatingtime of timing element C₄/R₄ begins to lapse. After that, capacitor C₄is recharged, in some instances by a reoccurrence of the detectorsignal. At the end of the following renewed start of the lapsing of thetiming element's operating time, for example at the instant 1.5 sec. inFIG. 3, the channel separation is continually raised to the optimalvalue.

In the embodiment of the present invention illustrated in FIG. 4, thetemporary reduction in the channel separation following a multi-pathreception period is readjusted with the aid of a channel-separationstage, which controls a field-strength-dependent phased mono-stereotransition. When working with a stereo radio receiver equipped with sucha circuit, in addition to the automatic mono switch-over stage 10, thetiming element 12 controls a changeover switch 13, which is looped intothe input of the circuit for the field-strength-controlled, phasedmono-stereo transition, which is normally linked to the field-strengthmeasuring output 14 of tuner 2. Changeover switch 13 is connected,during or following a multi-path reception situation, for the durationof the operating time of timing element 12, to a transducer 15, whichsimulates a fixed field strength, in response to which, for example, thephased mono-stereo transition adjusts itself to 12 dB.

According to an embodiment of the present invention a circuit for aphased mono-stereo transition, which under known methods heretofore isonly adjusted dependent upon the field-strength measuring signal, istuned for a time, even given a maximum reception field strength, to areduced measure of the channel separation, by specifying a transducervalue that simulates a lesser field strength, after one has left amulti-path reception area.

The analog modules in the above described circuits can be replaced,without difficulty, by digitally working processors, when the MPX signalis adapted to be tapped off in digital form at ZF stage 3, i.e. when thesignals are present in digital form at the inputs of the two audiochannels 7 and 8.

In the first case, the program of stereo processor 6 is supplemented bya program along the lines of the first embodiment, for processing theMPX signal. In the second case, the program of stereo decoder 6 issupplemented for a phased mono-stereo transition, along the lines of thesecond embodiment.

In both programs, after the operating time of the timing elements haselapsed, the counter content, which held the reduced (retained) degreeof channel separation during the operating time of the timing elements,is incremented in the direction of (i.e., to effect) an increase in thechannel separation.

What is claimed is:
 1. A stereo radio receiver comprising: a changeoverswitch; a detector for a multi-path reception of at least one signalfrom a stereo radio transmitter, the detector controlling the changeoverswitch to switch from a stereophonic reproduction having a channelseparation to a monophonic reproduction in response to the multi-pathreception, the detector controlling the changeover switch for a durationof the multi-path reception; a channel-separation stage; and a timingelement being switched on by the detector; wherein, when the timingelement operates and after the multi-path reception ends, the timingelement effects the stereophonic reproduction via the channel-separationstage for reducing the channel separation to a predetermined value thatis smaller than a maximum value of the channel separation, wherein thetiming element controls a phase shifter situated in a MPX signal path ofthe stereo radio receiver.
 2. A stereo radio receiver comprising: achangeover switch; a detector for a multi-path reception of at least onesignal from a stereo radio transmitter, the detector being operable tocontrol the changeover switch to switch from a stereophonic reproductionhaving a channel separation to a monophonic reproduction in response tothe multi-path reception, the detector being operable to control thechangeover switch for a duration of the multi-path reception; achannel-separation stage; and a timing element being switched on by thedetector; wherein, when the timing element operates and after themulti-path reception ends, the timing element is operable to effect thestereophonic reproduction via the channel-separation stage to reduce thechannel separation to a predetermined value that is smaller than amaximum value of the channel separation; a strength measuring unit beingswitched off by the changeover switch; and a circuit for providing aphased mono-stereo transition which is field strength dependent, thecircuit including an input circuit; wherein: in the input circuit of thecircuit for providing the phased mono-stereo transition, the timingelement controls the changeover switch; and for a transition time, thechangeover switch is operable to provide a fixed diminished fieldstrength.
 3. A stereo radio receiver comprising: a changeover switch; adetector for a multi-path reception of at least one signal from a stereoradio transmitter, the detector controlling the changeover switch toswitch from a stereophonic reproduction having a channel separation to amonophonic reproduction in response to the multi-path reception, thedetector controlling the changeover switch for a duration of themulti-path reception; a channel-separation stage; a timing element beingswitched on by the detector; and digitally operating signal processorspositioned in signal paths and including a digital counter, the digitalcounter having a content for determining the channel separation;wherein, when the timing element operates and after the multi-pathreception ends, the timing element effects the stereophonic reproductionvia the channel-separation stage for reducing the channel separation toa predetermined value that is smaller than a maximum value of thechannel separation, wherein, when the timing element operates, thecontent of the digital counter is maintained at an adjustablepredetermined value, and wherein, after the timing element stopsoperating, the content of the digital counter is continuously modifiedto increase the channel separation.
 4. A stereo radio receivercomprising: a changeover switch; a detector for a multi-path receptionof at least one signal from a stereo radio transmitter, the detectorcontrolling the changeover switch to switch from a stereophonicreproduction having a channel separation to a monophonic reproduction inresponse to the multi-path reception, the detector controlling thechangeover switch for a duration of the multi-path reception; achannel-separation stage; and a timing element being switched on by thedetector; wherein, when the timing element operates and after themulti-path reception ends, the timing element effects the stereophonicreproduction via the channel-separation stage for reducing the channelseparation to a predetermined value that is smaller than a maximum valueof the channel separation, wherein, after the timing element stopsoperating, the timing element continuously increases the channelseparation to an optimal value.
 5. A stereo radio receiver comprising: achangeover switch; a detector for a multi-path reception of at least onesignal from a stereo radio transmitter, the detector being operable tocontrol the changeover switch to switch from a stereophonic reproductionhaving a channel separation to a monophonic reproduction in response tothe multi-path reception, the detector being operable to control thechangeover switch for a duration of the multi-path reception; achannel-separation stage; and a timing element being switched on by thedetector; wherein, when the timing element operates and after themulti-path reception ends, the timing element is operable to effect thestereophonic reproduction via the channel-separation stage to reduce thechannel separation to a predetermined value that is smaller than amaximum value of the channel separation; wherein the timing element isoperable to control a phase shifter situated in a MPX signal path of thestereo radio receiver; wherein the channel separation stage includes aphase shifter, the phase shifter including: a first capacitor; and aseries circuit connected in parallel to the first capacitor, the seriescircuit including a second capacitor and a circuit closing device havinga control input circuit, the control input circuit including the timingelement, the timing element further including an RC combination; and thetiming element receives an output signal from the detector.
 6. Thestereo radio receiver according to claim 2 or 5, wherein thepredetermined value is 12 dB.
 7. The stereo radio receiver according toclaim 2 or 5, wherein the timing element controls a phase shiftersituated in an MPX signal path of the stereo radio receiver.
 8. Thestereo radio receiver according to claim 2 or 5, further comprising:digitally operating signal processors positioned in signal paths andincluding a digital counter, the digital counter having a content fordetermining the channel separation, wherein, when the timing elementoperates, the content of the digital counter is maintained at anadjustable predetermined value, and wherein, after the timing elementstops operating, the content of the digital counter is continuouslymodified to increase the channel separation.
 9. The stereo radioreceiver according to claim 2 or 5, wherein, after the timing elementstops operating, the timing element continuously increases the channelseparation to an optimal value.
 10. A stereo broadcast radio receivercomprising: a changeover switch; a timer; and a detector to detectmultipath reception of a stereo broadcast radio transmitter, thedetector being operable to control the changeover switch to change overfrom stereophonic reproduction to monophonic reproduction when themultipath reception occurs; wherein: the changeover switch is operableto cause the monophonic reproduction only for a duration of themultipath reception; and the timer is actuated by the detector and,after the end of the multipath reception and during its active time,uses a channel separation stage to fix a stereo channel separation to apredetermined reduced level between a minimum stereo channel separationand a maximum stereo channel separation to provide the stereophonicreproduction with reduced stereo channel separation; another changeoverswitch in an input circuit of a circuit to provide the mono-stereochangeover; and a field strength measuring arrangement to measure afield strength; wherein: the stereo broadcast radio receiver is operableto provide a smooth mono-stereo changeover that is a function of thefield strength; the timer is operable to control another changeoverswitch to switch field strength measuring arrangement off and to preseta fixed, lower field strength for a changeover time.
 11. The stereobroadcast radio receiver according to claim 10, wherein thepredetermined reduced level is 12 dB.
 12. The stereo broadcast radioreceiver according to claim 10, wherein a phase shifter switchable bythe timer is in an MPX signal path of the stereo broadcast radioreceiver.
 13. The stereo broadcast radio receiver according to claim 12,wherein: the phase shifter includes: a first capacitor; an on-switchassociated with a control input circuit, and connected in a seriescircuit with the first capacitor; a second capacitor connected inparallel with the series circuit; the timer includes aresistive-capacitive circuit arrangement; and the detector is operableto supply an output signal to the control input circuit of theon-switch.
 14. A stereo broadcast radio receiver according to claim 10,further comprising: a digital signal processor in a signal path todetermine the stereo channel separation level by a digital count;wherein the digital count is fixed at an adjustable and predeterminedvalue during an active time of the timer, and is continuously variableto increase the stereo channel separation level during an inactive timeof the timer.
 15. The stereo broadcast radio receiver according to claim10, wherein the timer is operable to continuously increase the stereochannel separation to an optimum level after an active time of the timerhas elapsed.
 16. The stereo broadcast radio receiver according to claim10, wherein an active time of the timer is started again by the detectorcoming into action again during a first active time.
 17. A stereo radioreceiver comprising: a changeover switch; a detector for a multi-pathreception of at least one signal from a stereo radio transmitter, thedetector controlling the changeover switch to switch from a stereophonicreproduction having a channel separation to a monophonic reproduction inresponse to the multi-path reception, the detector controlling thechangeover switch for a duration of the multi-path reception; achannel-separation stage; a timing element being switched on by thedetector; a strength measuring unit being switchable off by thechangeover switch; and a circuit to provide a phased mono-stereotransition which is field strength dependent, the circuit including aninput circuit; wherein: in the input circuit of the circuit forproviding the phased mono-stereo transition, the timing element isoperable to control the changeover switch; and for a transition time,the changeover switch is operable to provide a field strength; when thetiming element operates and after the multi-path reception ends, thetiming element effects the stereophonic reproduction via thechannel-separation stage for reducing the channel separation to apredetermined value that is smaller than a maximum value of the channelseparation.
 18. A stereo radio receiver comprising: a changeover switch;a detector for a multi-path reception of at least one signal from astereo radio transmitter, the detector controlling the changeover switchto switch from a stereophonic reproduction having a channel separationto a monophonic reproduction in response to the multi-path reception,the detector controlling the changeover switch for a duration of themulti-path reception; a channel-separation stage; and a timing elementbeing switched on by the detector; wherein, when the timing elementoperates and after the multi-path reception ends, the timing elementeffects the stereophonic reproduction via the channel-separation stagefor reducing the channel separation to a predetermined value that issmaller than a maximum value of the channel separation, wherein thechangeover switch is operable to provide a fixed field strength for amono-stereo transition.
 19. A stereo radio receiver comprising: achangeover switch; a detector for a multi-path reception of at least onesignal from a stereo radio transmitter, the detector controlling thechangeover switch to switch from a stereophonic reproduction having achannel separation to a monophonic reproduction in response to themulti-path reception, the detector controlling the changeover switch fora duration of the multi-path reception; a channel-separation stage; atiming element being switched on by the detector; a circuit to provide afield-strength-dependent, phased mono-stereo transition, the circuitincluding an input circuit, wherein the changeover switch is looped intothe input circuit to provide the field-strength-dependent, phasedmono-stereo transition; wherein, when the timing element operates andafter the multi-path reception ends, the timing element effects thestereophonic reproduction via the channel-separation stage for reducingthe channel separation to a predetermined value that is smaller than amaximum value of the channel separation.
 20. The stereo receiveraccording to claim 19, further comprising: a transducer to simulate afixed field strength, wherein the changeover switch is connected, one ofduring and following a multi-path reception situation, to the transducerfor a duration of an operating time of the timing element.
 21. A stereoradio receiver comprising: a changeover switch; a detector for amulti-path reception of at least one signal from a stereo radiotransmitter, the detector being operable to control the changeoverswitch to switch from a stereophonic reproduction having a channelseparation to a monophonic reproduction in response to the multi-pathreception, the detector being operable to control the changeover switchfor a duration of the multi-path reception; a channel-separation stage;and a timing element being switched on by the detector; wherein, whenthe timing element operates and after the multi-path reception ends, thetiming element is operable to effect the stereophonic reproduction viathe channel-separation stage to reduce the channel separation to apredetermined value that is smaller than a maximum value of the channelseparation, wherein an operation of the timing element is restarted inresponse to a renewed activation of the detector during a firstpredetermined operating time, wherein the timing element controls aphase shifter situated in an MPX signal path of the stereo radioreceiver.
 22. A stereo radio receiver comprising: a changeover switch; adetector for a multi-path reception of at least one signal from a stereoradio transmitter, the detector being operable to control the changeoverswitch to switch from a stereophonic reproduction having a channelseparation to a monophonic reproduction in response to the multi-pathreception, the detector being operable to control the changeover switchfor a duration of the multi-path reception; a channel-separation stage;a timing element being switched on by the detector; and digitallyoperating signal processors positioned in signal paths and including adigital counter, the digital counter having a content for determiningthe channel separation; wherein, when the timing element operates andafter the multi-path reception ends, the timing element is operable toeffect the stereophonic reproduction via the channel-separation stage toreduce the channel separation to a predetermined value that is smallerthan a maximum value of the channel separation, wherein an operation ofthe timing element is restarted in response to a renewed activation ofthe detector during a first predetermined operating time, wherein, whenthe timing element operates, the content of the digital counter ismaintained at an adjustable predetermined value, and wherein, after thetiming element stops operating, the content of the digital counter iscontinuously modified to increase the channel separation.
 23. A stereoradio receiver comprising: a changeover switch; a detector for amulti-path reception of at least one signal from a stereo radiotransmitter, the detector being operable to control the changeoverswitch to switch from a stereophonic reproduction having a channelseparation to a monophonic reproduction in response to the multi-pathreception, the detector being operable to control the changeover switchfor a duration of the multi-path reception; a channel-separation stage;and a timing element being switched on by the detector; wherein, whenthe timing element operates and after the multi-path reception ends, thetiming element is operable to effect the stereophonic reproduction viathe channel-separation stage to reduce the channel separation to apredetermined value that is smaller than a maximum value of the channelseparation, wherein an operation of the timing element is restarted inresponse to a renewed activation of the detector during a firstpredetermined operating time, wherein, after the timing element stopsoperating, the timing element continuously increases the channelseparation to an optimal value.